Indirectly heated cathode having portions with different thermal relations with a heater



A ril 7, 1970 w. SCHMIDT 3,505,557 INDIRECTLY HEATED CATHODE HAVING PORTIONS WITH DIFFERENT THERMAL RELATIONS WITH A HEATER F leCl NOV. 14, .1967

INVENTOR. WOLFGA N6 scnmor BY -W//- AGENT United States Patent Int. Cl. H01j 1/20, 19/14 US. Cl. 313-337 6 Claims ABSTRACT OF THE DISCLOSURE An indirectly heated cathode for a magnetron having two portions, one of which has a better thermal contact with a support for the cathode than the other, both portions being heated by a common heater.

The invention relates to a cathode for a magnetron tube comprising at least one cathode support, one surface of which is disposed towards the heater or heaters and on the other surface of which is disposed at least one first electron emitting body which is rigidly secured to the cathode support and beside which i arranged at least one second electron emitting body which consists of a material other than that of the first body.

In manufacturing cathodes for a magnetron tube, which generally are dispenser cathodes, not only is it essential for the cathode to adhere firmly to the cathode support, but also steps must be taken to increase the useful life of the magnetron by shaping the cathode so as to have a special form, since especially in a magnetron the cathode is heavily loaded.

In the operation of such a magnetron the primary electrons emitted by the cathode follow spiral paths to the anode. The electrons which return to the cathode with increased energy bombard the cathode so that it emits secondary electrons. When striking the cathode these electrons give up their energy to its so that its temperature is increased. This process, the so-called electron back bombardment, continues during the operation of the magnetron and hence special steps must be taken to prevent overloading of the cathode. The electron back bombardment further i.e. the cathode load, depends upon the input power of the magnetron. Therefore the input power and hence the output power of the magnetron are determined by the load capacity of the cathode. The load capacity of the cathode in turn depends upon the efiiciency of the heat dissipation or cooling of the cathode. In dispenser cathodes the heat dissipation largely takes the form of thermal conduction since the temperatures are so low that there is no substantially heat transfer by radiation.

Widely different steps are known to provide cathodes having high current densities and high efliciencies, as described in German Patents 8,858,985, 1,047,321 and 1,200,444. Some of these Patents and also German Patent 879,872 describe cathodes which each have various sur faces.

It is also known to fonn cathodes with several zones in a manner such that the surface of the cathode disposed towards the anode is divided into zones of different emissivities. Such a cathode for displaying letters on a screen is de cribed in United States 'Patent 3,131,328. Cathodes having surfaces of diflerent emissivities are known from the French Patents, 1,007,646; 945,839; 80,996; 1,277,518 and from the Britain Patent 643,656. Especially FIG- URES 1 and 2 of the British Patent 643,656 and FIG- URE 2 of the French Patent 1,277,518 show cathodes for ice magnetrons which consist of zones of different emissivities which are disposed on a common cathode support. The cathode support may consist of molybdenum The cathode zones having no or less emissivity consist of a material comprising the same substances but in proportions different from the proportion of the material of the emissive zone. However, use may also be made of a completely different material.

Although these known steps slightly reduce the inconvenient properties due to electron back bombardment, the useful life of the magnetron still is not satisfactory, since in spite of these known steps in operation the cathodes are excessively heated and consequently heavily loaded thermally.

Therefore it is an object of the invention so to divide the emissive portion of the cathode of a magnetron into a first body of high emissivity and one or more second bodies of smaller or low emissivity that the useful life of the magnetron i materially prolonged. According to the invention, in a cathode for magnetrons of the abovementioned kind this is achieved by securing the second body to the cathode support in at least thermally poor contact therewith. The first body is in satisfactory thermal contact with the support.

This for the first time enables the heating power to V be reduced and the warming-up period of a magnetron to be considerably shortened, since the use of the arrangement in accordance with the invention requires less heating power for the same output power of the magnetron because the mass of the first body is smaller. When the tube is switched into circuit, the first body begins to emit electrons. When the magnetron oscillates not only the first body but also the adjacent second body is struck by returning electrons and emits secondary electrons. Owing to parial evaporation of the emissive material from the first body and deposition of this material on the second body the latter may emit primary electrons after being heated by the impact of the returning electrons.

In a further embodiment of the invention, the second body may be spaced from the cathode support by a narrow gap so that the second body is more rapidly heated by the electron back bombardment. Since according to the invention the second body may be of slightly greater diameter than the first body, owing to the higher field strength prevailing at this area the emission will be concentrated onto the surface of the second body so that the load imposed on the first body by the emission and the electron bombardment is reduced. Owing to the poorer heat dissipation the temperature of the second body greatly exceeds the operating temperature of the first body and hence the second body dissipates the back-bombardment energy mainly by thermal radiation and not by thermal conduction. This again enables the output power of the magnetron to be increased and the useful life of the first body of the cathode to be prolonged.

Since only the first body is firmly secured to the cathode support with satisfactory thermal conductivity and since only that portion of the energy due to the electron back bombardment which is absorbed by this body has to be dissipated by thermal conduction via the cathode support, the magnetron power may be further increased because the first body is cooled more intensely than the second body.

According to the invention, the wall thickness of the cathode support may be reduced at the level of at least one of the second bodies since the heat dissipation, as set forth hereinbefore, of the second bodies mainly takes the form of thermal radiation and not of thermal conduction.

According to the invention, a further advantage is obtained when using a dispenser cathode, since in such a cathode the comparatively strong evaporation of the emissive material from the surface of the first body is reduced by the reduction of the active surface of the first body. Hence the troublesome deposition from vapour on the screening hoods generally incorporated in magnetrons and on the anode system is also greatly reduced so that the life of the magnetron is prolonged. Since according to the invention, as mentioned hereinbefore, the secon body has a greater diameter than the first body, emission material evaporated from the first body does not readily deposit on the screening hoods owing to the projecting edges of the second body.

The invention may best be described by comparing a known arrangement with an arrangement in accordance with the invention.

The invention will now be described with reference to an embodiment including two second bodies which is shown in the accompanying drawing, in which FIGURE 1 is a partially sectional view of a magnetron taken at the level of the cathode arrangement and FIGURE 2 is a view similar to that of FIGURE 1 of an arrangement in accordance with the invention.

In FIGURE 1, a heater 2 is secured between appropriately formed stay members 3 and 4 in a. tubular cathode support 1. On the outer surface of the cathode support 1, that is to say on the surface more remote from the heater 2, an electron emitting body 5 is provided on the support 1. This first body 5 is secured to the cathode support 1 in a manner such as to permit thermal conduction in the direction indicated by arrows 6. The heat flows in the tubular cathode support towards a cooling device (not shown). The magnetron further contains known screening hoods 7 and 8 and an anode 9.

The electrons emitted by the first body 5 follow spiral paths around the cathode towards the anode 9. Returning electrons impinge on the first body 5 so that this emits secondary electrons. Thus, the entire first body 5 is a struck by the electron bombardment. When the first body 5 is a dispenser cathode, as is usual in magnetrons, the evaporating emissive material is freely permitted to deposit on the projecting hood portions 10 and 11 as well, which also limits the useful life of the magnetron.

FIGURE 2 shows the arrangement in accordance with the invention. Corresponding parts are designated by like reference numerals. A comparison with the arrangement of FIGURE 1 shows at first sight that for the arrangement in accordance with the invention the heater 2 may be appreciably smaller.

Adjacent to this first body 5 second bodies 12 are provided the outer diameters of which exceed the outer diameter of the first body 5. As has been set forth hereinbefore, these likewise annular second bodies 12 screen the outer ends 10 and 11 of the screening hoods 7 and 8, and emissive material evaporating from the first" body 5 preferably deposits on the second bodies 12, so that the latter are also enabled to emit primary electrons during the operation of the magnetron. Again, the dissipation of heat from the first body 5 takes place by conduction, as is indicated by the arrows 6, since the first body 5 is secured to the cathode support 1 firmly and hence in good thermal contact therewith. In contrast therewith, the second bodies 12 are supported by the cathode support merely by means of support rings 13 with a gap 14 between the second" bodies 12 and the cathode support 1 so that heat cannot be dissipated from the second bodies 12 by thermal conduction to the cathode support 1 but can only be dissipated by thermal radiation.

Since the heat dissipation from the second bodies 12 takes place only by thermal radiation the Wall thickness of the cathode support 1 may be reduced, as is shown in the lower part of FIGURE 2 at 15, which in turn reduces the mass of the cathode support to be heated.

It is of particular advantage if in accordance with the invention the first body 5 is a dispenser cathode which mainly consists of tungsten, the second bodies 12 at least mainly consisting of molybdenum.

What I claim is:

1. A cathode for a magnetron comprising at least one cathode support, one surface of which is disposed toward a heater, a first electron emitting body consisting of an electron-emissive material rigidly secured to and in relatively good thermal contact with the support and a second electron emitting body which consists of a material other than that of said first body, said second body being secured to the cathode support in relatively poor thermal contact therewith.

2. A cathode as claimed in claim 1, wherein a narrow gap separates the cathode support and said second body.

3. A cathode as claimed in claim 2 wherein the outer diameter of said second body is greater than that of said first body.

4. A cathode as claimed in claim 3 wherein the wall thickness of the cathode support is reduced at the level of said second body.

5. A cathode as claimed in claim 4 wherein the first body is a dispenser cathode consisting of a porous body of tungsten containing a source of the electro-emissive material.

6. A cathode as claimed in claim 1 wherein said second body consists of molybdenum containing a source of electron emissive material.

References Cited UNITED STATES PATENTS 2,869,012 1/1959 Muller 3l3-346 X 2,957,100 10/1960 Espersen et a1. 313346 3,192,436 6/1965 Jewart 313-337 X 3,244,929 4/1966 Kuhl 3l3-346 X FOREIGN PATENTS 943,535 12/ 1963 Great Britain.

1,277,518 10/1961 France.

1,306,999 10/ 1962 France.

1,143,931 2/ 1963 Germany.

JOHN HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner U.S. Cl. X.R.

Patent Dated April 7, 1970 LJaoLEsANLscHmm It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

(SEAL) Atteat:

Column Column Column Column Column Signed and sealed this 25 line 42,

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change "its" (first occurence) "8,858,985, should be -'-885, 8985,

"electro-emissive" should be -electron-emissive EdwardMFlewhorJx. Attesfingofficcr day of Auguspfl 70.

WILLIAM 1:. sum, JR. Gomissioner of Patents 

